Thermal detectors include a sensor that absorbs light energy and then transduces the resulting heat into a useful electrical signal related to the amount or type of light absorbed. Perhaps the most prominent current thermal detectors include microbolometers, which absorb light across a broad band of the infrared, usually the mid-wave infrared (MWIR, corresponding to wavelengths of roughly 3 microns to 5 microns) or long wave infrared (LWIR, roughly 8 microns to 14 microns) and then convert heat into a change in resistance. These devices are very popular in commercial uncooled imaging cameras. Their basic structure includes a small micromachined sensor plate connected to an underlying substrate by thin support beams. The support beams have a low thermal conductance so that large increases in the temperature of the sensor plate occur with small amounts of absorbed light. The sensor plate includes a resistor made of a material with a high magnitude temperature coefficient of resistance (TCR). One common TCR material in use is vanadium oxide, originally developed for microbolometers in the 1980's. A pulsed or continuous bias current is applied to the resistor and the absorbed light energy can be measured through the voltage response. Some other common thermal detector technologies include thermoelectric detectors where the heat from light is converted into a voltage using the Seebeck effect, and pyroelectric detectors where heat from absorbed light induces a voltage signal via a change in the internal polarization of a ferroelectric material.
There are a variety of noise sources that can limit the performance of a thermal detector. For a biased single-pixel detector, the most important of these include Johnson noise, 1/f noise, and thermal noise. Thermal noise originates from the fluctuations in the quanta of energy transferred to and from the detector. These quanta can take the form of either phonons if solid-state conduction dominates the heat transfer or photons if radiation dominates. Traditionally, radiation heat transfer has been considered the fundamental noise limit because even if all of the other noise sources are reduced by technological innovations, the photon fluctuations still remain due to Planck's Law.